Most scientific papers are written in English. Non-native English speakers must simultaneously learn English and their area of scientific expertise. This is an obstacle for non-native English speakers in studying science. The difficulty of English used in scientific papers varies from field to field. For example, the vocabulary used in the medical sciences seems apparently difficult for English learners. Which scientific fields use English that is difficult for English learners to what extent? There are few existing studies in this regard.In this study, we compare the readability of English papers in various scientific fields by constructing and applying state-of-the-art artificial intelligence-based automatic readability assessor trained using actual textual data obtained by testing language learners and collecting judgments of language teachers. In experiments, our automatic assessors confirmed the intuition that medical science papers tend to be difficult for English learners. Moreover, our automatic assessors successfully quantified which field is difficult for English learners to what extent.